Mail cannot be delivered to a certain mailbox firstname.lastname@example.org
Spam filter is enabled for the mailbox in Home > Domains > example.com > Mail > email@example.com and option to delete spam messages is set
Messages are removed due to strict spam filter settings
Set less strict rules for Spam Filtering for such mail account
Set option to mark spam messages as ***SPAM*** instead of deleting to avoid blind removal of non-spam messages.
Check the article SpamAssassin Spam Filter to get more information about Spam Filters management in Plesk